In image-capture systems such as cameras embedded in mobile phones, it is necessary to ensure an image-stabilization function, in particular for compensating any hand motion of the user.
For this purpose an optical device is sought, capable of deviating an incident beam in an angular range of the order of ±1° at a frequency of between 5 and 30 Hz.
There are optical devices based on actuatable membranes capable of deviating an incident beam and/or varying the focal length.
FIG. 1A illustrates an optical device comprising a membrane 1 deformable on contact with a constant volume of fluid 4 and an actuation device 5 of a region 1a of the membrane located between a peripheral area 1c at the level of which the membrane is anchored in or on a support 3, and a central part 1b of said membrane. The main function of the actuation device is to chase some of the volume of fluid 4 of the periphery towards the center of the optical device. The configuration illustrated in FIG. 1A corresponds to the optical device at rest, that is, without electrical voltage being applied to the actuation device.
Such a fluid displacement principle varies a focal distance of the optical device by exerting symmetrical actuation about the central region of the membrane, as illustrated in FIG. 1B, or deviates an optical beam by exerting dissymmetrical actuation, as illustrated in FIG. 1C.
In FIG. 1B, the actuation device bends uniformly over the entire peripheral region of the membrane.
Deformation of the central part 1b of the membrane caused by displacement of the fluid induces focal length variation of the device.
In FIG. 1C, the actuation device does not bend uniformly over the entire actuation region 1a of the membrane, but instead presents different deflections according to placement on this actuation region. This configuration can for example be obtained by placing several actuators in the actuation region 1a of the membrane. Deformation of the membrane causes focal length variation and angular deviation (angle α) of the incident beam relative to the position at rest.
FIG. 1D illustrates a configuration of the optical device in which the position at rest (not shown) is divergent and non-uniform bending of the actuation device produces only deflection of the incident beam, without focal length variation.
The aptitude of the optical device to procure double variation of the focal length and of the angular deviation is sought to ensure the image-stabilization function in the cameras or other on-board optical systems.
There are also optical devices comprising two deformable membranes coupled mechanically by a constant volume of fluid trapped between them. An incident light beam is therefore intended to pass through the first membrane, the fluid then the second membrane, each membrane forming a diopter.
Document JP 11-1332110 describes such an optical device which comprises an actuation device for one only of both membranes, the other membrane able to deform freely as a function of the fluid pressure applied.
In this device, the activated membrane has a rigid central part, substantially non-deformable, and a deformable actuation region. As the actuators of the actuation device are capable of bending in two opposite directions, this device ensures the two functions of deviation and variation in focal length. To ensure the deviation function, the activated membrane must pivot without driving fluid to the second membrane which is suppler to avoid any focal length variation. For this purpose, the actuators are activated such that the central part of the membrane rises at one side and lowers at the other with the same range to ensure that the volume is preserved at its level. When preservation of the volume is no longer ensured at the level of the activated membrane, the fluid driven towards the non-activated membrane causes deformation of said membrane and consequently variation in focal length of the optical device.
In the event where the membranes can be assimilated into two plane diopters, the optical device can be assimilated into a prism.
Prismatic deviation, specifically the angle D between the beam entering the prism and the beam leaving the prism is given by the formula: D=(n−1)×A, where n is the optical index of the prism and A is the angle between the two faces of the prisms via which the beam enters and leaves.
To maximize deviation for a given optical index, the angle A between the two faces of the prism (therefore between the two membranes) thus has to be maximized.
The aim therefore is to design an optical device capable of increasing possible deviation relative to that of existing optical devices.